Quantitative determinations of cations, anions and electochemically active organic compounds can be carried out by voltammetric methods of analysis, such as for instance polarography, voltammetry, adsorption voltammetry, inverse voltammetry or chronoamperometric methods.
The analysis of a set of sample solutions can be done in a batch-like fashion or by using flow-through cells.
In the batch-type method, the sample solutions are transferred to sample vessels with a volume of about 1 to 50 ml. Subsequent analysis of the samples is carried out using a working electrode (mercury drop(ping) electrode), an auxiliary electrode and a reference electrode, all three of which are submerged into the sample solution. A disadvantage of this method and apparatus is that before refilling the sample vessels with subsequent sample solutions the sample vessels must be emptied and cleaned which requires large, additional volumes of sample solutions and is time consuming. In addition, the instrumentation for automatically changing sample solutions becomes costly and cumbersome. If inverse or adsorption voltammetry or other electrochemical methods involving enrichment of the electroactive species onto the working electrode are applied, it is necessary to stir the sample solution to obtain an effective enrichment. After the enrichment step, the stirring is stopped and the actual measurement of the electrochemically active species is performed once the sample solution has become quiescent since the measurement is based on a purely diffusion controlled process and convection currents in the sample solution interfere with the diffusion process. A disadvantage of using a large volume of sample solution is that the quiescent state is only reached several minutes after the stirring has been stopped.
Improvements are obtained if flow-through cells are used in which the sample solution flows through a narrow channel. Smaller volumes of sample solutions are required and the removal of the sample solution from the channel after analysis as well as the cleaning of the cell and subsequent refilling with the next sample solution is easily and quickly performed. Wall jet adapters (FIG. 10) do not belong to the flow-through cells and have a limited suitability, especially for inverse voltammetric methods, because of the reasons mentioned above.
W. W. Kubiak presents a review of flow-through cells with mercury drop(ping) electrodes in Electroanalysis, vol. 1, pp 379-388 (1989). Among others, a flow-through cell is described in which the channel is essentially horizontal and the mercury working electrode is placed vertically onto the channel. It is pointed out that the diameter of the channel should not be too small if a mercury dropping electrode is employed because the removal and collection of used mercury becomes a problem. With a handling mercury drop electrode this problem is less serious. In the state-of-the-art flow-through cells channel cross-sectional areas of usually 2 mm.sup.2 or more are employed.